1. Field of the Invention
This invention relates to lithography. More specifically, this invention relates to a scanning framing blade apparatus.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Examples of substrates include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle is used to transfer a desired pattern onto a substrate. The reticle is formed of a material transparent to the lithographic wavelength being used, for example glass in the case of visible light. The reticle has an image printed on it. The size of the reticle is chosen for the specific system in which it is used. For example, a reticle six inches by six inches and one-quarter inch thick can be used. During lithography, an illumination source illuminates a reticle that is disposed on a reticle stage. This illumination exposes an image onto a substrate that is disposed on a substrate stage. The image exposed onto the substrate corresponds to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the substrate. These changes correspond to the features projected onto the substrate during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the substrate during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the substrate, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the substrate.
An illumination source in a lithography process projects an illumination beam. The cross section of an illumination beam that is incident on a reticle is referred to herein as an illumination slot. Often, an illumination slot is narrower than an image printed on a reticle. In this case, scanning techniques are employed to project an entire reticle image onto a substrate. Scanning techniques involve scanning a reticle across an illumination slot to allow the full reticle pattern to be exposed onto a substrate that is simultaneously being scanned. Such techniques involve reticle stages and substrate stages that move with respect to the illumination source.
Scanning techniques must employ measures to prevent certain types of exposures, such as pin hole exposures, onto a substrate. Pin holes are small translucent defects that exist in opaque border areas surrounding a reticle image (also referred to as a field pattern). Pin hole exposures occur when illumination passes through one or more pin holes existing on a reticle. Border areas commonly have pin holes. As a reticle is scanned across an illumination slot, moments occur when portions of the border area are within the illumination slot. If pin holes exist in the border area, these pin holes will create unintentional exposures onto a substrate.
One measure taken to prevent pin hole exposure is the application of materials such as chrome to border areas. Materials such as these cover pin holes, thereby preventing pin hole exposure during scanning. Unfortunately, such measures are costly.
Another measure taken to prevent pin hole exposure involves the use of framing blades. Framing blades are physical objects having surfaces that obstruct areas of a reticle from an illumination slot. Common framing blade implementations mount framing blades on a reticle stage, thereby enabling the mounted framing blades and a reticle to move in unison as they pass across an illumination slot. Drawbacks with this approach exist, however. For instance, the mounting of framing blades can place excessive weight on a reticle stage.
Another drawback associated with the mounting of framing blades on a reticle stage involves the adjustment of reticle field sizes. A reticle field is the area of a reticle that is exposed onto a substrate. In addition to preventing pin hole exposure, framing blades are used to establish the width of a reticle field. Often, it is desirable to expose only a portion of a reticle's image area onto a substrate. In these cases, the field width is narrower than the reticle's image area. Moreover, during the operation of a lithography process, it is sometimes required to change a reticle's field width between scans. When framing blades are mounted on a reticle stage, changing a reticle's field width is a slow process because the reticle stage needs to be stopped to allow physical repositioning of the mounted framing blades.
What is needed is a synchronized framing blade apparatus that avoids the above drawbacks and is synchronized with the scanning operation.